The present invention relates to a hologram scanner which scans an object by using a laser beam diffracted by a hologram.
A bar-code reader is used in a market control system in a supermarket or the like. Such a bar-code reader reads a bar-code printed on a commodity and inputs the information data to a computer to control the operation of the supermarket. A hologram scanner is used as a bar-code scanner of the bar-code reader.
FIG. 1 is a view of a hologram scanner used as a bar-code reader according to the prior art. The scanner comprises a laser source 2, a beam expander 3, a hologram disc 4 comprising a plurality of hologram facet elements, a motor 5 for driving the hologram disc 4, a mirror 9 having a throughhole 9a at the center thereof for passing a laser beam, a condensor lens 10, and an optical detector 11, within an outer box 1. A laser beam from the laser source 2 passes through the throughhole 9a of the mirror 9 and is diffracted by a rotating hologram disc 4 so that the laser beam scans a bar-code 12 printed on a commodity 13, as a scanning beam 7 through a window 6. The laser beam 7 irradiated onto the bar-code 12 is scattered and a part 8 of the scattered light returns toward the hologram disc 4. The scattered light 8 is diffracted by the hologram 4, reflected by the mirror 9, and detected by the detector 11 through the lens 10.
Each component, such as the laser source 2, the hologram disc 4, the motor 5, the mirror 9; and the detector 11, of the hologram scanner is individually attached to the outer box 1 of the bar-code reader, together with the other components such as a control circuit, an interface unit and a power source. Such a structure is not compact and is inconvenient to handle, which causes difficulties when assembling the bar-code reader since the positioning of the parts of the hologram scanner is not easy.
The hologram disc 4 of the prior art scanner comprises a disc plate located perpendicularly to the rotational axis thereof. Therefore, the diffraction angle .theta. (FIG. 2) of the scanning beam 7 must be large for elongating the scanning line traced by the scanning point A with respect to the rotational angle of the hologram disc 4. However, if the diffraction angle .theta. is enlarged, the amount of the scattered light 8 received by the hologram disc 4 decreases, which results in the degradation of the reliability of the detection.
The reading ability of the scanner is upgraded as the laser beam is strengthened. However, the eyes of the operator or customer may be damaged if the laser beam is excessively strengthened. The laser beam strength must not exceed the safety standards for the human eye. The prior art method for enhancing the allowable laser beam strength in accordance with the safety standards is illustrated in FIG. 3. The laser beam for scanning in one direction is divided into two beams which are separated from each other by more than 7 mm at the outside of the scanning window 50' (d&lt;7 mm), by using hologram facets having slightly different diffraction angles. The two separated beams (dash line and solid line) scan in the same direction, e.g., in the direction perpendicular to the drawing sheet. By using two separated beams in one scanning direction, the strength of each of the beams can be enhanced, even if they continuously scan one after the other in the same direction, since they are deemed to be independent of each other. However, in such an arrangement, the width of the window 50' formed in the cover plate 49' disposed over the glass plate 48' must be enlarged to allow the passage of the two separated beams, which can lead to accidental damage of the glass plate by the article to be scanned above the window 50'.